Cross posting shows full content even though excerpt is selected

we want to show extracts of posts from the forum under the texts in the blog, but not the full text. For this we choose the setting "Cross posting | excerpt". Unfortunately the complete forum text is shown instead of an excerpt. How can this be changed? What could we have done wrong?

With the setting "Custom" nothing appears where you could choose a custom setting, and the whole text is shown here as well.

Please select the Customvalue, then go to the current post/page click on the "Update" button in post/page editor section. Then scroll a little down, you'll find the "Cross-posting Custom Content" textarea, just copy and paste the part of the content you'll need to cross-post.

OK, thanks for your patience. I don't want to overdo it with suggestions :-), but it would be convenient to automate it, for example the first 40, 50 or 60 letters of a post. We run a member site and want to show how much is going on in front of the password barrier - to attract new users and to feed Google.

If we do this via the excerpt function of Wordpress, I have to hire a new worker to do it ...